Convert CONFIG_CMD_MMC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_MMC

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ls1021aiot_qspi_defconfig b/configs/ls1021aiot_qspi_defconfig
index dba33e2..cb73a04 100644
--- a/configs/ls1021aiot_qspi_defconfig
+++ b/configs/ls1021aiot_qspi_defconfig
@@ -12,6 +12,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GPT=y
+CONFIG_CMD_MMC=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_EXT2=y